Hydrocarbon conversion processes utilizing supported heteronuclear cluster complex catalysts

ABSTRACT

Hydrocarbon conversion processes wherein hydrocarbon feedstreams are contacted with heteronuclear noble metal catalysts are improved by the use of supported heteronuclear noble metal cluster catalysts prepared from novel supported heteronuclear noble metal cluster complexes. The heteronuclear noble metal cluster complexes used as catalyst precursors are selected from the group consisting of 
     (pyridine) 2  Pt[Ir 6  (CO) 15  ], 
     (pyridine) 2  Pt[Ir 2  (CO) 7  ], 
     ((C 6  H 5 ) 3  P) 2  Pt[IR(CO) 3  P(C 6  H 5 ) 3  ] 2 , 
     (pyridine) 2  Pt[Ru 3  (CO) 12  ] 
     ((C 6  H 5 ) 3  P) 2  Rh(CO)[Ir(CO) 4  ], 
     (pyridine) 2  Pt[Rh(CO) 2  (P(C 6  H 5 ) 3 ) 2  ] 2 . 
     Heteronuclear noble metal cluster catalysts prepared by the reduction of supported heteronuclear noble metal cluster complexes exhibit greater activity, and lower coking rates for numerous hydrocarbon conversions than conventionally prepared supported mixed noble metal catalysts of the same nominal metals concentration.

DESCRIPTION OF THE INVENTION

This invention is directed to improved hydrocarbon conversion processes wherein a selected hydrocarbon feedstream is contacted with a supported heteronuclear noble metal cluster catalyst under suitable conditions thereby effecting the desired change in the feedstream, the improvement comprising using as the catalyst a supported heteronuclear noble metal cluster catalyst prepared from supported heteronuclear noble metal cluster complexes which heteronuclear noble metal cluster complexes are selected from the group consisting of

(pyridine)₂ Pt[Ir₆ (CO)₁₅ ],

(pyridine)₂ Pt[Ir₂ (CO)₇ ],

((C₆ H₅)₃ P)₂ Pt[Ir(CO)₃ P(C₆ H₅)₃ ]₂,

(pyridine)₃ Pt[Ru₃ (CO)₁₂ ]

((C₆ H₅)₃ P)₂ Rh(CO)[Ir(CO)₄ ],

(pyridine)₂ Pt[Rh(CO)₂ (P(C₆ H₅)₃)₂ ]₂

and the supports are selected from the group consisting of anhydrous refractory inorganic metal oxides and carbon, preferably alumina in either powder or extrudate form. The heteronuclear noble metal cluster catalysts retain the metal stoichiometry of the starting heteronuclear noble metal cluster complexes. In this way catalysts of precisely known metal ratios are obtained. Further, by preparing catalysts from the preformed heteronuclear noble metal cluster complexes, better control over the dispersion and distribution of the heterometal atoms is obtained in the final catalyst than in conventional catalyst preparation methods.

In the practice of the process of the instant invention the metal loadings on the support is within the range of 0.001 to 5.0 wt.% of platinum, the other metal component being present at a wt% in conformity with its stoichiometry. In the case of the rhodium-iridium complex, the metal loading is within the range of 0.001 to 5.0 wt. % of rhodium, the other metals being present in conformity with the stoichiometry. Preferably the metal loadings are within the range of 0.01 to 3 wt.% platinum (or rhodium), the other metal being present in conformity with the stoichiometry.

Expressed in other terms, the total heterometal loading is within the range of 0.002 to 35 wt.% based on total catalyst weight, the amount of each metal present being determined by the stoichiometry of the heteronuclear noble metal complex precursor. Preferably, the total heterometal loading is in the range of 0.002 to 10 wt.% based on total catalyst weight.

Further, the catalysts used in the instant improved hydrocarbon conversion process may contain a halogen such as chlorine and/or fluorine. The amount of chlorine and/or fluorine present, if and when used, is in the range of 0.001 to 3.0 wt.%, preferably 0.1 to 2.5 wt.%, most preferably 0.5 to 2.0 wt.%. The halogen can be added either after preparation of the catalyst is finished or it can be added to the support prior to catalyst preparation. The halogen of choice is chlorine.

The hydrocarbon conversion processes which are improved by the use of the supported catalyst prepared from supported heteronuclear noble metal carbonyl complexes include but are not limited to (1) naphtha reforming, (2) dehydrocyclization, (3) olefin formation by dehydrogenation of paraffinic feedstocks, (4) hydrogenation of olefinic and aromatic feedstocks, and (5) paraffin isomerization.

In a naphtha hydroforming (reforming) process, a substantially sulfur-free naphtha stream that typically contains about 20-80 vol.% paraffins, 20-80 vol.% naphthenes, and about 5 to 20% aromatics, and boiling at atmospheric pressure substantially between about 80° and 450° F., preferably between about 150° and 375° F., is brought into contact with the catalyst system of the present invention in the presence of hydrogen. The reactions typically take place in the vapor phase at a temperature varying from about 650° to 1000° F., preferably about 750° to 980° F. Reaction zone pressures may vary from about 1 to 50 atmospheres, preferably from about 5 to 25 atmospheres.

The naphtha feedstream is generally passed over the catalyst at space velocities varying from about 0.5 to 20 parts by weight of naphtha per hour per part by weight of catalyst (w/hr/w), preferably from about 1 to 10 w/hr/w. The hydrogen to hydrocarbon molal ratio within the reaction zone is maintained between about 0.5 and 20, preferably between about 1 and 10. During the reforming process, the hydrogen employed can be in admixture with light gaseous hydrocarbons. Since the hydroforming process produces large quantities of hydrogen, a recycle stream is employed for admission of hydrogen with the feed.

The catalysts can also be employed, as previously stated, in the formation of aromatic compounds by contacting the catalyst with suitable paraffins or naphthenes wherein the reaction proceeds at a temperature varying between about 700° and 1000° F. and a pressure of less than about 10 atmospheres in the presence of hydrogen. The catalysts of this invention can also be employed to promote the isomerization of ethylbenzene to xylenes by contacting ethylbenzene with the catalyst at a temperature varying from about 400° to 1100° F. at elevated pressures in the presence of hydrogen.

The conditions for the hydrogenation reactions vary widely and are well known to those skilled in the art; broadly the following conditions may be utilized: temperatures 150°-1000° F.; pressure 1 atm-12,000 psig; feed rate 0.2-100 V/Hr/V; H₂ treat 2,000-10,000 SCF/B.

The use of transition metal carbonyl complexes for the preparation of supported metal catalysts is a new area of research of particular interest to catalytic chemists. Monomeric metal carbonyl complexes such as Ni(CO)₄ have been used to prepare supported catalysts, e.g. Ni/Al₂ O₃ catalysts (see Parkyns, N. D., "Proceedings 3rd International Congress on Catalysis", (W. H. M. Sachtler, G. C. A. Schuit & P. Zweitering, Eds.) p. 194, North-Holland, Amsterdam, 1965). The use of metal cluster complexes (defined here as complexes containing more than one transition metal of a single type) as supported catalyst precursors has, however, only recently been suggested (see Anderson, J. R. and Mainwaring, D. E., J. Catalysis, 35, 162 (1974), Anderson, J. R., Elmes, P. S., Howe, R. F. and Mainwaring, D. E., J. Catalysis, 50, 508 (1977) and Ichikawa, M., J. Chem. Soc. Chem. Comm., 11 (1976). The utilization of heteronuclear cluster complexes (defined here as complexes containing at least two different transition metals) as precursors for the generation of supported heterometallic catalysts has likewise received very little attention. Furthermore, heterometallic cluster complexes in which the heterometal atoms are noble metals (as opposed to First Period Group VIII metals) have not heretofore been employed as supported catalyst precursors.

The instant invention relates to a new and improved preparation of supported heterometal noble metal catalysts containing at least two different noble metals selected from the group Ru, Rh, Ir and Pt via the usage of novel, especially formulated heterometal noble metal cluster complexes. The heterometal noble metal cluster catalysts derived by the deposition, and subsequent reduction of preformed heterometal noble metal cluster complexes have been found to be exceptionally active and selective hydrocarbon conversion catalysts.

Conventional heterometallic noble metal catalysts are prepared by the impregnation of a suitable support with an aqueous solution containing the appropriate metal salts followed by drying and reduction. The multiple salt impregnation step can be either sequential or simultaneous. In such preparations the clustering of metal atoms to form heteronuclear clusters on the support surface is not well-defined. In conventional aqueous impregnations of supports, pH changes often occur within the salt solution. Such pH changes can lead to metal aggregation processes which in turn yield poorly dispersed supported metal phases (e.g. Ru/Al₂ O₃ and Pt/SiO₂ -Al₂ O₃) in the finished catalyst. The formation of supported heteronuclear clusters also requires that the different metal salts are absorbed onto the support surface at equivalent rates. This situation is known not to generally occur (see Harriott, P., J. Catalysis, 14, 43 (1969)). In general then, the chemistry leading to complete and uniform clustering (alloying) of heterometal atoms on support surfaces is not well understood and/or readily controlled.

In concert with the proper choice of support, the use of heteronuclear cluster complexes as heterogeneous catalyst precursors offers numerous advantages. These include: (1) the preparation of supported catalysts exhibiting well-dispersed and uniform heterometal cluster phases, (2) the supported heterometal clusters may retain the unique geometries of the starting heteronuclear cluster complexes, (3) extremely efficient clustering of the heterometals on the support surface is likely since the precursor complexes are held together by strong heterometal metal-metal bonds, and (4) the surface heterometal stoichiometry can be systematically varied by employing precursor cluster complexes of different known compositions, stoichiometries and structures.

The number of heteronuclear noble metal cluster complexes commercially available or reported in the literature is extremely limited. The instant specification discloses new noble metal cluster complexes which have been discovered. The ultimate use of such heteronuclear noble metal cluster complexes is for the preparation of novel supported heteronuclear noble metal cluster catalysts which find utility in a wide variety of hydrocarbon conversion reactions. The novel heteronuclear noble metal cluster complexes disclosed in the instant application were in general prepared by metathetical exchange reactions.

A. Preparation of Heteronuclear Noble Metal Cluster Complexes

Air sensitive reactions and catalyst preparations using air sensitive materials were carried out in a "dry box" employing a purified nitrogen atmosphere. Solvents used in heteronuclear noble metal complex and supported heteronuclear noble metal cluster catalyst preparations were dried and degassed by standard techniques.

(1) (pyridine)₂ Pt[Ir₆ (CO)₁₅ ]

This new composition was prepared by the metathetical reaction between Na[Ir₆ (CO)₁₅ ] (see L. Malatesta, G. Caglio and M. Angoletta, Chem. Comm., 532 (1970)) and cis -(pyridine)₂ PtCl₂ (see G. B. Kauffman, Inorg. Syn., 7, 251 (1963)) in THF solution. The complex is a dark red solid and is readily soluble in moderately polar organic solvents such as THF or methylene chloride. The complex is rapidly decomposed in air but is stable indefinitely under nitrogen. A pure product was obtained (75% yield) by recrystallizing the crude material from a 50/50 mixture of THF/n-pentane. The complex was found to contain (%): C, 17.3; H, 1.37; N, 1.45; Pt, 10.1; Ir, 60.1; O, 13.3; Cl, 0.02; calculated %: C, 15.6; H, 0.52; N, 1.45; Pt, 10.1; Ir, 59.9; O, 12.5; Cl, nil. The infrared bands in the carbonyl region for the (pyridine)₂ Pt[Ir₆ (CO)₁₅ ](hereafter identified as PtIr₆) complex in the solid state and in solution in THF are listed in Table I. These spectra are characterized by a pair of strong bands near 2010 and 1975 cm⁻¹. These bands are readily assigned to terminal carbonyl stretching frequencies. Weak to medium intensity bands in the 1780-1730 cm⁻¹ region are assigned to bridging carbonyl groups. The infrared spectra of the Ir₆ (CO)₁₅.sup.⊖⊖ dianion (Na or NH₄ salt) in THF solution are characterized by a strong terminal carbonyl band near 1970 cm⁻¹ and bridging carbonyl modes near 1780 and 1730 cm⁻¹. Upon interaction of the Ir₆ (CO)₁₅.sup.⊖⊖ dianion with (pyridine)₂ Pt.sup.⊕⊕, the terminal bands of the Ir₆ (CO)₁₅.sup.⊕⊕ dianion are shifted by about 40 cm⁻¹ to higher wave numbers. Such a frequency shift is consistent with a transfer of electron density from the Ir₆ (CO)₁₅.sup.⊖⊖ dianion onto the Pt.sup.⊕⊕ center. This electronic rearrangement implies the formation of a covalent bond (bonds) between the Pt atom and the Ir₆ (CO)₁₅ cluster. The carbonyl band positions of the bridging carbonyl groups are not appreciably changed by this interaction. A reasonable structure for the PtIr₆ cluster complex is presented in FIG. I where only the metal atoms are shown for simplicity. In the proposed structure the Pt atom is assumed to be bonded to a face on the octahedral Ir₆ (CO)₁₅ cluster. The coordination about the Pt atom is completed by two pyridine molecules. The local symmetry of the Pt atom is probably close to square planar. Within the Ir₆ (CO).sub. 15 cluster each Ir atom is assumed to possess two terminal carbonyl groups. The remaining three carbonyl groups must then occupy bridging positions on alternating faces of the Ir₆ octahedron.

The proposed structure for the PtIr₆ cluster complex, however, does not in any way restrict the well-defined stoichiometric relationship between the Pt and Ir atoms in the discrete molecule and is presented merely by way of explanation.

(2) (pyridine)₂ Pt(Ir₂ (CO)₇)

This novel cluster complex was prepared for the first time by the metathetical reaction between Na(Ir(CO)₄) and trans-(pyridine)₂ PtCl₂ (see G. B. Kauffman, Inorg. Syn. 7, 251 (1963)) in THF solution. The Na[Ir(CO)₄ ] intermediate was prepared by the sodium amalgam reduction of either (Ir(CO)₃ Cl)_(x) or Ir₄ (CO)₁₂ in THF solution under 70-75 psig of carbon monoxide. The Pt-Ir complex is a red-brown solid and is readily soluble in moderately polar organic solvents such as THF or methylene chloride. The complex is extremely air sensitive, but can be stored under nitrogen. A pure product was obtained (86% yield) by recrystallizing the crude material from a 50/50 mixture of THF/n-pentane. The complex was found to contain (%): Pt, 19.6; Ir, 42.5; calculated (%): Pt, 20.8; Ir, 41.0.

The unique (pyridine)₂ Pt[Ir₂ (CO)₇ ] complex (hereafter identified as PtIr₂) was prepared by the metathetical reaction between trans-(py)₂ PtCl₂ and Na[Ir(CO)₄ ] in THF solution. The product from this reaction was expected to be trans(pyridine)₂ Pt[Ir(CO)₄ ]₂ (see equation 1). An analogous trans(pyridine)₂ Pt[Co(CO)₄ ]₂ complex, prepared according to equation 1 except that ##STR1## Py=pyridine Na[Co(CO)₄ ] was used in place of Na[Ir(CO)₄ ], exhibits carbonyl bands at 2025s, 1975s and 1900 sh cm⁻¹ (see R. G. Pearson and J. Dehand, J. Organometallic Chem., 16 485 (1969)). A structurally similar Ir complex would be expected to display carbonyl bands in the same region. The PtIr₂ complex was found, however, to display four carbonyl bands in the terminal region (2040-1900 cm⁻¹) in the solid state and in solution in THF. In addition to the strong terminal carbonyl bands a strong bridging carbonyl band near 1730 cm⁻¹ was present in the solid and solution spectra of the PtIr₂ complex. A triangular (diphos)Pt[Co₂ (CO)₇ ] has been prepared by the reaction of (diphos)PtCl₂ (chloride ions cis) with Na[Co(CO)₄ ] in THF solution (equation 2). The (diphos)PtCo₂ cluster complex exhibits carbonyl bands (KBr pellet) at ##STR2## P P=bis(diphenylphospheno)ethane=diphos 2049s, 2010s, 1952 vs, 1970sh and 1729s cm⁻¹. The 1729 cm⁻¹ band has been assigned to the bridging carbonyl group. (See J. Dehand and J. F. Nennig, Inorg. Nucl. Chem. Letters, 10, 875 (1974). The infrared spectrum of solid PtIr₂ is very similar to that displayed by the (diphos)PtCo₂ complex. This suggests that the PtIr₂ complex adopts an analogous structure (FIG. 2). To obtain this structure, it must be assumed that an initially formed trans-(pyridine)₂ Pt[Ir(CO)₄ ]₂ complex (intermediate) undergoes a trans to cis isomerization. Accompanying the isomerization is the expulsion of one mole of carbon monoxide and the formation of an Ir-Ir bond. The formation of an Ir-Ir bond is suggested to preserve the 18 electron rule for the Ir atoms. The terminal carbonyl bands exhibited by PtIr₂ are centered about 90 cm⁻¹ to higher energy than the Ir(CO)₄.sup. ⊖ anion. This shift to higher energy suggests that the electron density on the Ir atoms in the PtIr₂ complex is lower than in the Ir(CO)₄.sup. ⊖ anion. The distribution of electronic charge indicated by the infrared implies the formation of a covalent interaction between the two Ir atoms and the Pt atom. The geometry about the Pt atom is probably close to square-planar with the two pyridine molecules and the two Ir atoms occupying mutually cis positions. Each Ir atom is assumed to be six-coordinate with approximate octahedral symmetry.

3 (C₆ H₅)₃ P)₂ Pt[Ir(CO)₃ P(C₆ H₅)₃ ]₂

This new composition was prepared by the exchange reaction between trans-((C₆ H₅)₃ P)₂ PtCl₂ and Na[Ir(CO)₃ P(C₆ H₅)₃ ] in THF solution. The Na[Ir(CO)₃ P(C₆ H₅)₃ ] intermediate was prepared by the reduction of a THF solution of ((C₆ H₅)₃ P)₂ Ir(CO)Cl with sodium amalgam under 70 psig carbon monoxide at 140° F. for 24 hours (see J. P. Collman, F. D.Vastine and W. R. Roper, J. Amer. Chem. Soc., 90, 2282 (1968). The Pt-Ir complex is a yellow-orange solid and is readily soluble in common organic solvents. The complex is moderately stable in air. A pure product was obtained (69% yield) by recrystallizing the crude material from benzene. The complex was found to contain (%): Pt, 10.8; Ir, 21.4; Cl, nil; calculated (%): Pt, 10.9; Ir, 21.4; Cl, nil. The complex is essentially monomeric in benzene solution (M. W. found: 1640, M. W. calculated: 1790).

The ((C₆ H₅)₃ P)₂ Pt[Ir(CO)₃ P(C₆ H₅)₃ ]₂ complex (hereafter identified as L₂ PtIr₂) was found to exhibit in the solid state and in THF solution five to six terminal carbonyl bands in the 2080-1900 cm⁻¹ region. The band maxima is centered near 1960 cm⁻¹. This maxima is located 90-100 cm⁻¹ to higher energy than the major band displayed by the [Ir(CO)₃ P(C₆ H₅)₃ ].sup.⊖ anion in THF solution. The higher energy bands exhibited by L₂ PtIr₂, in contrast to the anion, suggests the presence of relatively covalent Pt-Ir bonds. The proposed covalent Pt-Ir interaction is supported by the fact that a covalent Hg[Ir(CO)₃ P(C₆ H₅)₃ ]₂ complex exhibits high energy carbonyl bands at 1945 and 1985 cm⁻¹. The formation of covalent Pt-Ir bonds is further suggested by the complexity of the carbonyl bands displayed by L₂ PtIr₂. If local C_(3v) symmetry rules were obeyed by the Ir(CO)₃ P(C₆ H₅)₃ moiety, only two infrared active carbonyl bands would be allowed. The presence of additional bands is assumed to result from vibrational coupling between two trans Ir(CO)₃ P(C₆ H₅)₃ groups through the Pt atom. The two weak bands in the 1800-1750 cm⁻¹ region most likely arise from a small Ir₂ Pt₂ (P(C₆ H₅)₃)₄ (CO)₆ impurity. A proposed structure for the L₂ PtIr₂ complex is shown in FIG. 3.

The three carbonyl groups and phosphine ligand bonded to each Ir atom are omitted for simplicity. The symmetry about the Pt atom is probably close to square planar. Each Ir atom is assumed to be five coordinate with the phosphine and Pt atom occupying apices. Three equivalent carbonyl groups are assumed to complete the trigonal plane. The inability of L₂ PtIr₂ to form a closed PtIr₂ cluster as observed for (pyridine)₂ Pt[Ir₂ (CO)₇ ] is likely the result of steric interference. The bulky phosphine must force the [Ir(CO)₃ P(C₆ H₅)₃ ] moieties to occupy transpositions about the Pt atom. Such a geometry would not allow a close approach of the Ir atoms which would facilitate the formation of a closed PtIr₂ system containing an Ir-Ir bond.

4 (Pyridine)₃ Pt[Ru₃ (CO)₁₂ ]

This new complex was prepared by the metathetical reaction between cis-(pyridine)₂ PtCl₂ and Na₂ [Ru₃ (CO)₁₂ ] in THF solution in the presence of excess pyridine. The Na₂ [Ru₃ (CO)₁₂ ] intermediate was prepared for the first time by the sodium amalgam reduction of Ru₃ (CO)₁₂ under 70 psig carbon monoxide. The red-brown Pt-Ru complex is moderately stable in air, as no decomposition was noted upon standing in air for several hours. The complex is readily soluble in THF, but exhibits limited solubility in nonpolar solvents such as benzene. A pure product was obtained by recrystallizing the crude material from a 20/80 mixture of THF/n-pentane. The Pt-Ru complex was found to contain (%): C, 29.3; H, 1.94; N, 4.00; O, 19.4; Pt, 17.7; Ru, 27.6; calculated (%): C, 30.3; H, 1.41; N, 3.92; O, 17.9; Pt, 18.2; Ru, 28.3.

The Ru₃ (CO)₁₂.sup.⊖⊖ dianion intermediate (Na salt) exhibits five carbonyl bands in the 2030-1890 cm⁻¹ region. The major terminal carbonyl band is centered near 1950 cm⁻¹. A weak to medium band at 1649 cm⁻¹ is due to a small unknown impurity. The Ru₃ (CO)₁₂.sup.⊖ ⊖ dianion is a new species and is probably generated by the reduction of a single Ru-Ru bond in the starting Ru₃ (CO)₁₂ complex. Complete reduction of the Ru₃ (CO)₁₂ cluster would have been expected to yield Na₂ Ru(CO)₄ (carbonyl band near 1800 cm⁻¹ expected). The effect of localized charge is demonstrated by the isoelectronic series Fe(CO)₄.sup.⊖⊖ (1788), Co(CO)₄.sup. ⊖ (1883), and Ni(CO)₄ (2057) where the carbonyl stretching frequency (cm⁻¹) increases with decreasing charge on the metal center. The major band exhibited by the Ru₃ (CO)₁₂.sup.⊖⊖ dianion is intermediate in energy between those shown by Co(CO)₄.sup. ⊖ and Ni(CO)₄. This suggests that the two negative charges are delocalized over the three Ru atom framework. The effective charge is then probably close to two thirds of an electron per Ru atom. Upon complexation of the Ru₃ (CO)₁₂.sup.⊖⊖ dianion with a Pt.sup.⊕⊕ center, the infrared band contour remains nearly the same but the band maxima is shifted about 50 cm⁻¹ to higher energy (1950→2000 cm⁻¹ shift). This frequency shift suggests the formation of covalent Pt-Ru bonds. Weak bands at 1804 and 1763 cm⁻¹ in the spectrum of (pyridine)₃ Pt(Ru₃ (CO)₁₂) (hereafter identified as PtRu₃) suggests the presence of a small amount of unknown impurity. Based upon elemental analyses, infrared spectroscopy and the well known tendency for Pt.sup.⊕⊕ complexes to adopt a square planar configuration, a reasonable structure for the PtRu₃ complex is presented in FIG. 4. The four terminal carbonyl groups bonded to each Ru atom have been omitted for simplicity. The Pt atom is assumed to be square planar with two pyridine molecules and the two end Ru atoms of the Ru₃ (CO)₁₂ moiety occupying mutually cis-positions. Such a structure is rationalized by assuming displacement of chloride ions from cis-(pyridine)₂ PtCl₂ by the Ru₃ (CO)₁₂.sup.⊖⊖ dianion. The third mole of pyridine is assumed to be present as a molecule of crystallization to preserve a square planar Pt atom. Each Ru atom is assumed to possess four terminal carbonyl groups. Relatively intense bands in the 1800-1700 cm⁻¹ region would be expected if the PtRu₃ complex contained a significant number of bridging carbonyl groups. The proposed bonding scheme represents the first example of a closed PtRu₃ ring system.

5. ((C₆ H₅)₃ P)₂ Rh(CO)[Ir(CO)₄ ]

This novel heteronuclear noble metal cluster complex was prepared by the metathetical reaction between ((C₆ H₅)₃ P)₂ Rh(CO)Cl and NaIr(CO)₄. The solid heteronuclear Rh-Ir complex is dark red and is readily soluble in moderately polar organic solvents such as THF, benzene or methylene chloride. The complex is rapidly decomposed in air but is stable indefinitely under nitrogen. A pure product (90% yield) was obtained by recrystallizing the crude material from a 50/50 mixture of THF/n-pentane. The complex was found to contain (%): Rh, 10.5; Ir, 19.8; Cl, 0.01; calculated (%): Rh, 10.7; Ir 20.0; Cl, nil.

The starting ((C₆ H₅)₃ P)₂ Rh(CO)Cl and Na[Ir(CO)₄ ] complexes exhibit carbonyl stretching frequencies at 1965s and 1896vs and 1863sh cm⁻¹, respectively, in THF solutions. The new ((C₆ H₅)₃ P Rh(CO)[Ir(CO)₄ ] (hereafter identified as RhIr) complex exhibits carbonyl bands at 2042 w, 1995sh, 1987 vs, 1962 sh and 1932 m cm⁻¹ in THF solution. The shift in carbonyl band positions to higher wave numbers upon complexing the Ir(CO)₄.sup.⊖ anion to a Rh.sup.⊕ center as well as the complexity of the spectrum, suggests the formation of a covalent Rh-Ir bond. The absence of carbonyl bands below 1900 cm⁻¹ further suggests the presence of terminal carbonyl groups only. A reasonable structure for the Rh-Ir complex is shown in FIG. 5. The bonding about the Rh atom is likely to be either square planar or slightly distorted toward tetrahedral symmetry while the local symmetry about the Ir atom is most probably C_(3v).

6. (pyridine)₂ Pt[Rh(CO)₂ (P(C₆ H₅)₃)₂ ]₂

This new cluster complex can be readily prepared by allowing trans-(pyridine)₂ PtCl₂ to react with Na[Rh(CO)₂ (P(C₆ H₅)₃)₂ ] in THF solution. The Na[Rh(CO)₂ (P(C₆ H₅)₃)₂ ] salt is easily obtained by reducing ((C₆ H₅)₃ P)₂ Rh(CO)Cl with a 1% Na amalgam in the presence of 40-60 psig carbon monoxide in THF solution (see page 10, lines 20-21). The cluster complex can be purified by recrystallizing the crude product from a 20/80 mixture of THF/n-pentane.

The novel heteronuclear noble metal cluster complexes disclosed herein were found to exhibit physical and chemical properties typical of covalent metal complexes. The cluster complexes are maintained in discrete molecular geometries by covalent metal-metal bonds. The heteronuclear metal-metal bonds remain intact in solution in non-polar solvents. By varying metal stoichiometry, heteronuclear cluster complexes of different molecular structures were prepared.

These new heteronuclear noble metal cluster complexes will be used as precursors for the preparation of supported heteronuclear cluster catalysts which in turn find utility as active and selective hydrocarbon conversion catalysts.

B. Preparation of Supported Catalysts

Base case or standard catalysts were prepared by the aqueous impregnation of η-Al₂ O₃ powders or γ-Al₂ O₃ extrudates with the appropriate standardized transition metal salt solutions. Heteronuclear noble metal catalysts were prepared by co-impregnation with standardized aqueous salt solutions. Both incipient wetness (I.W.) and adsorption (ADS) impregnations were carried out. η-Al₂ O₃ powders (BET surface areas 170-220 m² /gm) were prepared by calcining beta alumina tri-hydrate at 1100° F. under air for five hours. The γ-Al₂ O₃ extrudates (BET surface areas 180-200 m² /gm) were commercially available samples. The freshly impregnated catalysts were routinely dried overnight at 250°-280° F. and then reduced under 20% H₂ /He (500-1200 cc/min) for several hours at an elevated temperature (790°-930° F.).

Supported heteronuclear noble metal cluster catalysts, defined as catalysts prepared by the deposition of preformed heteronuclear noble metal cluster complexes onto ceramic supports, were prepared under an inert atmosphere (N₂) since the precursor complexes were in many cases sensitive to air and moisture. The ceramic supports were dried at elevated temperatures 660°-1020° F. and outgassed prior to use to minimize hydrolysis of the starting complexes. Carefully dried organic solvents (THF or methanol), containing known amounts of the appropriate heteronuclear noble metal cluster complex, were employed as impregnation media. Both incipient wetness and adsorption impregnations were employed. Since the precursor complexes were chlorine free, the ceramic supports were in some cases prechlorided (aqueous HCl). Alternatively, acidity was built into the supported cluster catalysts by treating reduced cluster catalysts with aqueous HCl. Freshly impregnated cluster catalysts were vacuum dried (10⁻⁴ mm) at room temperature. The dry catalysts were then reduced with 20% H₂ /He (500-1200 cc/min), with careful exclusion of air, for several hours at an elevated temperature (790°-930° F.). These reduction conditions were found to be sufficient to completely decompose the precursor heteronuclear noble metal cluster complexes on the support surface.

Analyses of selected Al₂ O₃ supported standard and heteronuclear noble metal cluster catalysts are given in Table II. Analyses were performed on reduced and thus stabilized catalyst samples.

Characterization of Heteronuclear Noble Metal Cluster Complexes Supported on Alumina Infrared Spectroscopic Studies

The preparation of supported heterometallic cluster complexes was outlined above. Prior to reduction, vacuum dried supported heteronuclear noble metal cluster catalysts were subjected to infrared measurements. Spectra in the carbonyl region were recorded on Nujol mull samples sandwiched between KBr salt plates. Infrared samples were prepared in the strict absence of oxygen and moisture.

A (pyridine)₂ Pt[Ir₆ (CO)₁₅ ] cluster complex on Al₂ O₃ catalyst (nominally 0.4% Ir/0.067% Pt) was found to exhibit five terminal carbonyl bands in the 2070-1970 cm⁻¹ region (see Table III). The major band is centered near 2020 cm⁻¹. A weak to medium band near 1800 cm⁻¹ is assigned to the vibration of bridging carbonyl groups. The similarity in carbonyl band shapes and band positions between the supported PtIr₆ cluster and a mull spectrum of the solid PtIr₆ complex (Table I) suggests that the complex remains intact on the Al₂ O₃ surface. The retention of bridging carbonyl bands further suggests the surface stability of Ir clusters. The supported PtIr₆ cluster is readily oxidized and undergoes rapid loss of carbonyl band intensity upon exposure to air. During the initial stages of oxidation new carbonyl surface species are produced. These species exhibit carbonyl band maxima near 2040 and 1630 cm⁻¹. The high wavenumber band suggests the presence of partially oxidized iridium carbonyl complexes. The low wavenumber band is assigned to an iridium carbonate surface species. Carbonate formation is presumably via oxidation of complexed carbon monoxide. A [(C₂ H₅)₄ N]₂ [Ir₆ (CO)₁₅ ] monometallic cluster on Al₂ O₃ catalyst (nominally 0.4% Ir) was found to display four or five bands in the terminal carbonyl region (2015-1925 cm⁻¹). Bridging carbonyl bands in the 1800-1700 cm⁻¹ region were also present in the spectrum. The similarity in carbonyl band positions between the supported Ir₆ anion and the anion in THF solution suggests that the Ir₆ cluster is stable on the Al₂ O₃ surface. The supported Ir₆ cluster is extremely air sensitive and rapidly loses carbonyl band intensity upon exposure to air. The intermediate oxidation products display carbonyl and carbonate bands at the same wavenumber positions as the supported PtIr₆ cluster complex. This result suggests that the sub-carbonyl species formed during oxidation of the suggested PtIr₆ cluster complex are associated with the iridium component.

The infrared spectrum of a (pyridine)₂ Pt[Ir₂ (CO)₇ ] cluster complex on Al₂ O₃ catalyst (nominally 0.32% Ir/0.16% Pt) displays four or five terminal carbonyl bands in the 2070-1970 cm⁻¹ region. The major terminal carbonyl band is situated near 2010 cm⁻¹. Several weak to medium bands in the bridging carbonyl region (1800-1750 cm⁻¹) are also clearly present in the spectrum of the supported PtIr₂ complex. The general shape of the infrared carbonyl bands of the supported PtIr₂ cluster complex was found to be very similar to those exhibited by the solid PtIr₂ complex. The major carbonyl band of the PtIr₂ cluster complex is, however, shifted about 30 cm⁻¹ to higher energy upon deposition on the Al₂ O₃ surface. This shift suggests that the PtIr₂ complex is interacting with the Al₂ O₃ support surface. The energy shift is consistent with the transfer of electron density from the PtIr₂ cluster to the support (the electron transfer may be to a Lewis acid center on the Al₂ O₃ surface). The bridging carbonyl band positions are also shifted to higher energy upon deposition of the PtIr₂ complex onto Al₂ O₃. In contrast to the PtIr₂ complex, the PtIr₆ complex did not exhibit a pronounced shift in its infrared upon deposition onto Al₂ O₃. These results may reflect the larger size of the PtIr₆ cluster within which a partially positive charge (generated by interaction with the Al₂ O₃ support) could be more readily distributed. As was found for the supported PtIr₆ cluster, the supported PtIr₂ cluster was rapidly oxidized in air.

An ionic K[Ir(CO)₄ ] complex on Al₂ O₃ catalyst (nominally 2% Ir) was found to display a rich infrared spectrum in the carbonyl region. The strong bands at 1892 and 1868 cm⁻¹ indicates the presence of ionic Ir(CO)₄ ⊖ surface species. Additional strong bands near 1995 and 1925 cm⁻¹ suggest the existence of partially oxidized iridium carbonyl complexes. The absence of bridging carbonyl bands suggests that clusters of iridium atoms are not present on the Al₂ O₃ surface. Upon exposure to air the carbonyl bands due to the Ir(CO)₄ ⊖ anion and the partially oxidized iridium carbonyl surface species are rapidly reduced in intensity. The oxidation process is accompanied by growth of carbonyl bands near 2040 and 1635 cm⁻¹. The 2040 cm⁻¹ band, as discussed before, is most likely due to an oxidized iridium carbonyl surface species. The low energy band is reasonably assigned to an iridium carbonate surface complex.

A (pyridine)₃ Pt[Ru₃ (CO)₁₂ ] cluster on Al₂ O₃ catalyst (nominally 0.33% Ru/0.22% Pt) was found to exhibit a mull spectrum very similar in detail to that displayed by the solid PtRu₃ complex. This result suggests that the PtRu₃ complex is not degraded on the Al₂ O₃ surface. The intermediate Ru₃ (CO)₁₂.sup.⊖⊖ dianion on Al₂ O₃ (nominally 2% Ru) was also found to be reasonably stable. In the case of the supported dianion (K salt) the major terminal carbonyl band near 2000 cm⁻¹ is about 50 cm⁻¹ to higher energy than the corresponding band of the dianion in THF solution. This suggests that the Ru₃ (CO)₁₂.sup.⊖⊖ dianion is partially oxidized by the underlying Al₂ O₃ support. The fact that the PtRu₃ complex is not appreciably oxidized by the support is in line with the relatively high air stability of this complex when compared to the anion.

Infrared measurements have shown that the heteronuclear noble metal cluster complexes remain intact on a dehydrated Al₂ O₃ surface. Upon air exposure the cluster complexes generally undergo rapid decomposition. This process is probably irreversible. When subjected to hydrogen reduction the organic portion of the cluster complexes is destroyed, leaving behind reduced heteronuclear noble metal clusters on the Al₂ O₃ surface. This novel method of catalyst preparation offers several potential advantages. Alloy formation on the Al₂ O₃ surface by the above procedure may be more favorable than by the conventional coimpregnation of monomeric, aqueous salt solutions. In the latter case good contacting between the supported metal atoms requires that both metal salts are equivalently adsorbed by the Al₂ O₃ support. This situation is known not to be generally obeyed. Metal-support interactions may, in some cases, drive heterometallic systems into a state where the metals approach monoatomic dispersion. Well dispersed metal phases, as is often found in low metal-loaded (<1%) catalysts, may disfavor surface alloy formation. The covalent metal-metal bonds present in heteronuclear noble metal cluster complexes may, in contrast, maintain the heteroatoms within bonding distance as the molecular complex is decomposed on the Al₂ O₃ surface. In essence, a supported bimetallic cluster complex may reduce as a single molecular entity, whereas, two supported monomeric salts would possess different reduction characteristics. The latter case could result in metallic phase separation. The decomposition of a heteronuclear noble metal cluster complex may also generate discrete heteronuclear crystallites on the support surface. We suggest that the reductive decomposition of a PtIr₆ cluster complex may yield uniform, three-dimensional metal clusters within which the basic octahedral symmetry of the starting complex is retained. The conventional coimpregnation of monomeric Pt and Ir salts in a 1 to 6 mole ratio would not be expected to generate a discrete three-dimentional, 1 to 6 clustering of Pt and Ir atoms. Available evidence suggests that well dispersed Pt/Al₂ O₃ catalysts possess pseudo two-dimensional (pill-box) crystallites. The Pt crystallites are flat and of uniform thickness on the Al₂ O₃ surface. Three-dimensional crystallites proposed to be present in heteronuclear cluster catalysts would possess a relatively higher number of unsaturated atoms than a pill-box cluster of metal atoms. The presence of unsaturated metal atoms would be predicted to modify catalytic reaction patterns. An a priori statement as to the direction of catalyst modification (activity and/or selectivity) cannot, however, be made with any certainty.

Hydrogen Chemisorption Studies

Selected supported Pt/Ir catalysts were subjected to hydrogen chemisorption measurements. The apparatus used for adsorption measurements of supported catalysts was a conventional glass vacuum system. A high precision, fused quartz Bourdon pressure gauge manufactured by Texas Instrument Company, Houston, Texas, was used for the H₂ adsorption measurements. Prior to adsorption measurements the catalysts were reduced in situ under flowing hydrogen (ca. 500 cc/min) for 2.0 hours at 750°-930° F. In some cases argon BET surface areas were also determined. The dispersions of standard and cluster deposited Pt/Ir catalysts, as measured by their characteristic H/M values, are compared in Table VI.

The standard catalysts (prepared with standardized H₂ MCl₆ acid (M=Ir or Pt) solutions) were found to display H/M ratios greater than unity. The departure from unity was found to be greater in the more dilute catalysts (compare catalysts 1 and 2). We assume that these high H/M ratios indicate complete dispersion of the supported metal phases. We further assume that hydrogen uptake in excess of that corresponding to an H/M=1 is associated with either hydrogen spillover or multiple hydrogen bonding to the metal crystallites. The method of impregnation, either incipient wetness or adsorption from an overstanding solution, did not measurably alter the H/M ratios of the standard catalysts.

Supported cluster complex catalysts were in most cases found to display H/M ratios near unity. The most serious departure from this value was found in the case of a higher loaded catalyst (catalyst 8) which exhibited an H/M ratio of 0.80. The apparent absence of hydrogen spill-over or multiple hydrogen bonding in supported heteronuclear noble metal cluster catalysts is not understood. We have assumed, however, that the metal phases in these catalysts are generally well dispersed. As was found for the standard catalysts, the mode of impregnation did not measurably affect the dispersion level of the supported heteronuclear noble metal cluster catalysts. The presence of chloride was also found not to have an affect on the dispersion level of the cluster catalysts of the instant invention.

Reduced PtIr₆ and PtIr₂ cluster catalysts were generally found to exhibit H/M ratios of near 1.0. The metallic phases present in these catalysts were completely amorphous to X-ray diffraction measurement. Standard Pt/Ir catalysts containing comparable metal loadings were found to display H/M ratios between 1.5 and 1.8. The uptake of hydrogen in excess of H/M=1.0 was assumed to be due to either hydrogen spillover onto the Al₂ O₃ support or to multiple hydrogen bonding to the crystallites. The absence of hydrogen spillover or multiple hydrogen bonding in Pt-Ir cluster catalysts is not understood. The dissimilar hydrogen adsorption capacities may reflect crystallite and/or metal-support interaction differences between standard and cluster Pt-Ir catalysts. For catalytic purposes we consider both the standard and cluster catalysts to be fully dispersed.

C. Hydrocarbon Conversion Reactions

Hydrocarbon conversion reactions were carried out in a 25 cc stainless-steel, fixed-bed, isothermal hydrotreating unit. The unit was operated in a single pass mode. Hydrogen was passed through deoxo and molecular sieve drying units prior to use. The hydrotreating unit is heated by a fluidized sand bath. The feed is delivered by a dual barrel Ruska Pump which allows continuous operation. Model compound reaction products were analyzed by in-line G.C. measurements. Gas chromatographic measurements were carried out in a Perkin-Elmer 990 Chromatograph using flame ionization detection. The column was 1/8 inch by 30 feet packed with a 20% SP-2100 substrate on a ceramic support.

n-Heptane dehydrocyclization experiments were carried out on unsulfided catalysts at 900° F. under 200 psig total pressure. A w/hr/w of 20 was employed. The H₂ /n-C₇ mole ratio was maintained at a 5.0-5.6 level. Under these conditions n-heptane is converted into 21-23 products, starting with methane and extending through the isomeric xylenes. Experiments were typically carried out for 100-200 hours.

Naphtha reforming experiments were carried out at 905°-912° F. under 200 psig total pressure. A w/hr/w of 2.26 was employed. Hydrogen was supplied at a 6000 SCF H₂ /bbl level. In situ reduced catalysts (930° F., H₂ (1150 cc/min), 2.0 hours) were sulfided to breakthrough with 0.5% H₂ S/H₂ (1.0 atm) at 700° F. The reformate was collected over ice and was analyzed for research octane number (RON). Off gases were monitored by wet test meter. Catalysts were typically maintained on feed for 200-300 hours. Relative catalysts activities (RCA) were calculated using equation 3: ##EQU1## where S.V.=space velocity and ^(Q) B and ^(T) B are empirical pilot plant correlations of the octane effect and temperature effect, respectively.

Spent catalysts from the above hydrocarbon conversion reactions were routinely analyzed for % carbon and % chlorine.

n-Heptane Dehydrocyclization Results

Screening studies singled out PtIr₆ cluster catalysts as being of particular interest for catalytic reforming. The results of n-heptane dehydrocyclization experiments carried out over 0.4% Ir/0.067%Pt/0.47%Cl/Al₂ O₃ catalysts are presented in Table V. A supported PtIr₆ cluster catalyst with added chloride is compared against a standard catalyst of the same nominal metals and chloride concentrations. The catalysts were maintained on feed for 210 hours so as to compare their deactivation rates. Catalyst activity was allowed to decline with time on feed (no temperature or space velocity corrections were made to maintain a constant conversion level). This mode of operation allows one to determine relative deactivation rates.

The decline in n-heptane conversion with time on feed is shown in FIG. 6. This plot clearly shows that under identical reaction conditions the PtIr₆ cluster complex maintains a significantly higher level of conversion than the standard catalyst. Toluene selectivity as a function of time curves are shown in FIG. 7. For lined out catalysts, between 100 and 200 hours on feed, the cluster catalyst displayed a constant selectivity to toluene of about 25%. During the same time period, however, the standard catalyst experienced about a 4% decline in toluene selectivity. Toluene selectivity as a function of n-heptane conversion curves are shown in FIG. 8. At n-heptane conversions between 70 and 77% the cluster catalyst consistently exhibits a higher (0.5-1.0%) toluene selectivity than the standard catalyst. FIGS. 9 and 10 are plots of C₄.sup.⊖ selectivity and i-C₇ selectivity versus n-heptane conversion, respectively. The PtIr₆ cluster catalyst was found to exhibit about a 1.5% higher selectivity to C₄.sup.⊖ products (at a given conversion level) than the standard catalyst. To counterbalance the higher toluene and C₄.sup.⊖ selectivities the PtIr₆ cluster catalyst demonstrated about a 2% lower i-C₇ isomerization selectivity than the standard catalyst. The spent PtIr₆ cluster and standard catalysts were found to contain 0.7 and 1.2% coke, respectively.

In an effort to understand the effect of cluster size on hydrocarbon conversion reactions, several 0.32% Ir/0.16% Pt catalysts were prepared by depositing (pyridine)₂ Pt[Ir₂ (CO)₇ ] onto Al₂ O₃. The chloride level was adjusted to 0.5% by the addition of aqueous HCl to a reduced PtIr₂ cluster catalyst. The results of n-heptane dehydrocyclization over a series of 1:2 Pt-Ir catalysts are compared in Table VI. The activity of a chlorided PtIr₂ cluster catalyst was found to be nearly the same as a standard catalyst. The chloride-free cluster catalyst was somewhat less active (˜5% lower conversion) than the chloride containing catalysts. Throughout the lifetime of the runs the cluster catalysts exhibited higher C₄.sup.⊖ selectivities (5-6%) than the standard catalyst. The PtIr₂ cluster catalysts were found to be less active isomerization catalysts than the standard catalyst. Within experimental error, the three 1:2 Pt-Ir catalysts exhibited equivalent selectivities to toluene. After 100 hours on feed the catalysts produced nearly equivalent amounts of coke.

A (pyridine)₃ Pt[Ru₃ (CO)₁₂ ] on Al₂ O₃ catalyst (nominally 0.33% Ru/0.22% Pt) was prepared. The results of n-heptane dehydrocyclization experiments over this catalyst are presented in Table VII. The PtRu₃ cluster catalyst was found to be considerably less active than a comparably loaded standard Pt/Ir catalyst. The catalyst demonstrated extremely high C₄.sup.⊖ and i-C₇ selectivities. A very poor toluene selectivity was shown by the catalyst and did not markedly improve upon increasing the temperature to 950° F. The overall selectivity pattern exhibited by the PtRu₃ cluster catalyst appears to be governed by the high hydrogenolysis activity of the Ru component.

n-Heptane Reaction Patterns

Alumina supported PtIr₆ cluster catalysts were found to exhibit markedly different n-heptane dehydrocyclization reaction patterns than conventionally prepared 1:6 Pt-Ir catalysts (see Table V). A PtIr₆ cluster catalyst in which the acidity level was balanced by the addition of chloride ion was found to be considerably more active than a standard catalyst (see FIG. 6). The activity advantage may result in part from a lower coke level on the PtIr₆ cluster catalyst. The lower coke make found with PtIr₆ cluster catalysts suggests that the useful lifetimes of these catalysts are substantially longer than standard Pt-Ir catalysts.

Under normal reforming conditions the dehydrocyclization of n-heptane produces a complex mixture of reaction products. These products can be conveniently divided into three groups (equation 4): ##STR3## Of these products, toluene, benzene and the isomeric xylenes are the most desirable because of their high research octane numbers (RON) and their use as gasoline blending agents. The C₄.sup.⊖ yield is to be minimized as these products represent feed losses. The C₅.sup.⊕ skeletal isomerization products are in general product credits as they possess considerably higher RON values than n-heptane. Under typical reforming conditions it has been found that product selectivities are not greatly influenced by catalyst acidity. The activity (as measured by n-heptane conversion) of cluster catalysts was, however, increased upon the addition of chloride ion.

The dehydrocyclization and isomerization of n-heptane are considered by numerous workers in the field to be dependent upon the dispersion of the metal component. In this sense these reactions can be considered to be demanding. The selectivity patterns exhibited by a chlorided PtIr₆ cluster catalyst were found to be substantially different than those exhibited by a standard 1:6 Pt-Ir catalyst containing an equivalent chloride level. Toluene selectivity as a function of time over these catalysts are presented in FIG. 7. As previously stated, between 100 and 200 hours on feed the PtIr₆ cluster catalyst displayed a constant toluene selectivity of about 25%. Throughout the lifetime of the run the standard catalyst experienced about a 4% decline in toluene selectivity. These results indicate that, in contrast to the standard catalyst, the PtIr₆ cluster catalyst maintained a constant toluene selectivity as conversion decreases. At n-heptane conversion levels between 70-77% the PtIr₆ cluster catalyst demonstrated a consistently higher (˜1%) toluene selectivity (see FIG. 8). At comparable conversion levels the PtIr₆ cluster catalyst produced a 1.5% higher C₄.sup.⊖ selectivity than the standard catalyst (see FIG. 9). The standard catalyst was found to exhibit about a 2.0% higher selectivity to i-C₇ products than the PtIr₆ cluster catalyst (see FIG. 10). The hydrocracking and isomerization selectivity differences shown by the standard and PtIr₆ cluster catalysts suggest that the metal crystallites are on the average smaller in the case of the standard catalyst since it is known that hydrocracking decreases and isomerization increases (for C₅ to C₇ paraffins) as the dispersion of Pt/Al₂ O₃ catalysts increases. The higher H/M ratios exhibited by standard Pt-Ir catalysts also suggest that these catalysts have smaller particles (fewer metal atoms in each crystallite) than the Pt-Ir cluster catalysts.

The enhanced dehydrocyclization selectivity demonstrated by the PtIr₆ cluster catalyst cannot, however, be completely correlated with a simple metal crystallite size (dispersion) argument. The dehydrocyclization selectivity differences may be due to subtle changes in crystallite geometries. The PtIr₆ cluster catalyst may contain small (11A crystallites based upon H/M values of around 1.0) three-dimensional, bimetallic clusters while the standard catalysts (H/M=1.5-1.8) may contain smaller (less than 10 A) two-dimensional bimetallic clusters. These proposed microstructure differences can be used to rationalize the dehydrocyclization advantage displayed by the PtIr₆ cluster catalyst. The presence of small three-dimensional clusters, as opposed to still smaller two-dimensional clusters, would favor dehydrocyclization since carbocyclic reaction intermediates would be preferentially generated on a less highly coordinated three-dimensional bimetallic cluster. Even though PtIr₆ cluster and standard 1:6 Pt-Ir catalysts are essentially 100% dispersed the cluster crystallite geometry and hence catalytic specificity exhibited by the two catalysts are markedly different.

Alumina supported PtIr₂ cluster catalysts were prepared in an attempt to ascertain if the size of the precursor heteronuclear cluster complex could influence hydrocarbon conversion reactions. For n-heptane dehydrocyclization PtIr₂ cluster catalysts (chlorided and chloride free) were found to produce substantially higher C₄.sup.⊖ selectivities than a standard 1:2 Pt-Ir catalyst (see Table VI). At comparable conversion levels this C₄.sup.⊖ selectivity debit amounted to about 5-6%. The high hydrocracking activity demonstrated by the chloride-free PtIr₂ cluster catalyst was not anticipated. This result suggests that the C₄.sup.⊖ products are generated primarily over metal sites (the quantity of support acid cracking is small). The mechanism whereby the metal clusters present on the PtIr₂ cluster catalysts produce higher gas makes than the standard catalyst is not known. As expected the activity of a PtIr₂ cluster catalyst was increased (˜5%) by the addition of chloride ion. The n-heptane conversion level of a chlorided PtIr₂ cluster catalyst was found to be comparable with that of the standard catalyst. Both PtIr₂ cluster catalysts demonstrated lower (6-7%) isomerization selectivities than the standard catalyst. These results cannot be correlated with catalyst acidity differences since the chlorided PtIr₂ cluster and standard catalyst contain nearly the same chloride concentrations. The higher hydrocracking and lower isomerization selectivities demonstrated by the PtIr₂ cluster catalysts suggest (as for the PtIr₆ cluster catalyst) that the average metal crystallite size is somewhat larger in these catalysts than in the standard catalyst. Within experimental error the three 1:2 Pt-Ir catalysts demonstrated equivalent toluene selectivities. The spent catalysts were found to produce nearly the same amounts of coke.

The precursor PtIr₂ cluster complex is most probably square-planar and may upon deposition and reduction on an Al₂ O₃ surface yield a two-dimensional (raft-like) clustering of Pt and Ir atoms. The simultaneous impregnation of Al₂ O₃ with a 2:1 mixture of Ir and Pt acid solutions may produce upon reduction structurally similar bimetallic sites. If the above process were in fact to occur during catalyst preparation the similar catalytic patterns (activity, toluene selectivity, coke make) shown by PtIr₂ cluster and standard 1:2 Pt-Ir catalysts can be rationalized.

The toluene selectivities exhibited a PtIr₆ cluster catalyst were found to be 5 to 7% higher than a PtIr₂ cluster catalyst (see Tables V and VI). The dehydrocyclization selectivity advantage demonstrated by the PtIr₆ cluster catalyst is too large to be accounted for by either the difference in Ir content or conversion level. Since the PtIr₆ and PtIr₂ cluster catalysts are both 100% dispersed the activity and dehydrocyclization selectivity advantage exhibited by the PtIr₆ cluster catalyst must reflect subtle differences in catalyst microstructure. Different supported microstructures can be rationalized if we assume that deposition of a PtIr₆ cluster complex generates a three-dimensional bimetallic cluster while a PtIr₂ cluster complex yields a two-dimensional bimetallic cluster. As discussed earlier, three-dimensional clusters should more readily catalyze the dehydrocyclization of n-heptane than two-dimensional clusters.

A reduced PrRu₃ cluster complex on Al₂ O₃ catalyst was found to be considerably less active for n-heptane dehydrocyclization than a comparable loaded Pt/Ir catalyst (see Table VII). The PtRu₃ cluster catalyst demonstrated high C₄.sup.⊖ and i-C₇ selectivities. A very low toluene selectivity was shown by the catalyst and this did not improve significantly upon increasing the reaction temperature from 900° to 950° F. The overall selectivity pattern exhibited by the PtRu₃ cluster catalyst appears to be governed by the high hydrogenolysis activity of the Ru component. At comparable n-heptane conversion levels a Pt only catalyst (0.3% Pt/0.6% Cl) would exhibit C₄.sup.⊖ and toluene selectivities of 10 and 15%, respectively. The poor activity and selectivity pattern displayed by the PtRu₃ cluster catalyst indicated that such catalysts would not likely prove to be useful in catalytic reforming applications, but would be useful hydrogenation catalysts.

Naphtha Reforming Results

The above n-heptane dehydrocyclization experiments have shown that several Pt-Ir noble metal cluster catalysts exhibit modified reaction patterns when compared to conventionally prepared Pt-Ir bimetallic catalysts.

The modified n-heptane reaction patterns displayed by noble metal cluster catalysts has been tentatively ascribed to the generation of unique Pt-Ir clusters on the Al₂ O₃ support surface. Such unique heterometallic clusters significantly alter reactions dependent upon catalyst microstructure (demanding reactions). These results suggest that Pt-Ir cluster catalysts may find particular application in catalytic reforming.

1:6 Pt-Ir Catalysts

The results of naphtha reforming over Al₂ O₃ -supported PtIr₆ and conventional 1:6 Pt:Ir catalysts (nominally 0.4% Ir/0.067% Pt/0.7% Cl) are presented in Table VIII. The conventional 1:6 Pt-Ir catalyst was in the form of 1/16 inch extrudate while the PtIr₆ cluster was in the form of 10/20 mesh. After 170 hours on feed the standard catalyst was found to line out at a relative catalyst activity (RCA) of about 190. The PtIr₆ cluster catalyst (prepared by depositing (Py)₂ Pt[Ir₆ (CO)₁₅ ] on η-Al₂ O₃) lined out at a RCA near 410. Thus, under the same set of reaction conditions the PtIr₆ cluster catalyst is at least 2.2 times more active than the conventional catalyst. The spent catalysts were found to contain essentially the same chloride loadings. The PtIr₆ cluster catalyst demonstrated a 30% lower coking rate than the conventional catalyst.

In an attempt to eliminate diffusional effects, which might account in part for the large naphtha reforming activity difference, PtIr₆ cluster and conventional catalysts were prepared on 1/16 inch Al₂ O₃ extrudate. The results of naphtha reforming over these catalysts are presented in Table IX. The RCA patterns exhibited by the conventional and PtIr₆ cluster catalysts were found to be in good agreement with those of the previous runs. After about 170 hours on feed the PtIr₆ cluster catalyst was found to be about 2.4 times more active for naphtha reforming than the conventional catalyst. After the same length of time on feed the activity difference found in the previous runs was close to 2.2. These results suggest that the form of a PtIr₆ cluster catalyst (1/16 inch extrudate or 10/20 mesh) does not measurably effect its activity. Also, the type of Al₂ O₃ (η or γ) had no apparent effect on catalyst performance.

Ir Only Catalysts

Several 0.4% Ir/0.7% Cl/Al₂ O₃ catalysts were prepared in an attempt to ascertain what contribution the Pt component had on naphtha reforming over 1:6 Pt-Ir catalysts. A Ir₆ cluster catalyst was prepared by depositing a [(C₂ H₅)₄ N]₂ [Ir₆ (CO)₁₅ ] cluster complex on η-Al₂ O₃. The results of naphtha reforming over Ir₆ cluster and conventional catalysts are presented in Table X. For the first 120 hours on feed the Ir₆ cluster and conventional catalysts exhibited nearly comparable naphtha reforming activities. After this period, the activity of the Ir₆ cluster catalyst rapidly declined reaching a RCA value of around 60 after 200 hours on feed. The spent catalysts were found to have comparable coke and chloride contents. Thus the rapid fall off in activity displayed by the Ir₆ cluster catalyst cannot be explained by either a large coke make or chloride loss. At about 160 hours on feed the activity of the Ir₆ cluster catalyst was only about 30% as great as that of a PtIr₆ cluster catalyst. The large activity advantage demonstrated by the PtIr₆ cluster catalyst cannot be accounted for by the presence of the platinum component. After about 160 hours on feed the conventional 0.4% Ir catalyst was found to be about 1.4 times more active for naphtha reforming than a conventional 0.4% Ir/0.06% Pt catalyst.

Composite RCA versus time on feed curves for the catalysts summarized in Tables VIII, IX and X are shown in FIG. XI. These curves demonstrate that the activity patterns exhibited by conventional and PtIr₆ cluster catalysts are very reproducible. The curve displayed by the PtIr₆ cluster catalysts clearly shows a 1.6 fold increase in activity between 60 and 120 hours on feed. This behavior is likely associated with a gradual loss of sulfur. After about 140 hours on feed both the conventional and PtIr₆ cluster catalysts exhibited lined out activities.

The nearly 2.4 fold activity advantage demonstrated by the PtIr₆ cluster catalyst is significant. Since the metal and chloride content of the conventional and PtIr₆ cluster catalysts are equivalent, the enhanced reforming activity demonstrated by the PtIr₆ cluster catalyst must be related to differences in catalyst microstructure (heterometallic cluster geometries).

During the catalytic runs outlined in Tables VIII, IX and X the C₅.sup.⊕ fraction was collected over ice and weighed. From this weight, a % weight yield was calculated from a knowledge of the weight of feed employed during timed collection periods. FIG. 12 presents curves of research octane number (RON) versus C₅.sup.⊕ liquid yield (wt.%) for the catalysts listed in Tables VIII, IX and X. At comparable RON values the conventional and PtIr₆ cluster catalysts demonstrate similar C₅.sup.⊕ liquid yield.

1:2 Pt-Ir Catalysts

The results of naphtha reforming over 0.32% Ir/0.16% Pt/0.7% Cl/Al₂ O₃ are summarized in Table XI. The PtIr₂ cluster catalyst was prepared by depositing a (Py)₂ Pt[Ir₂ (CO)₇ ] cluster complex on η-Al₂ O₃. Both the conventional 1:2 Pt-Ir and PtIr₂ cluster catalysts were in the form of 10/20 mesh. Curves of RCA versus time on feed are shown in FIG. 13. After about 200 hours on feed the PtIr₂ cluster catalyst was found to be about 1.6 times more active for naphtha reforming than the conventional 1:2 PtIr catalyst. The spend catalysts were found to display similar chloride levels. The spent PtIr₂ cluster catalyst contained a slightly higher coke content than the conventional catalyst. FIG. 14 presents curves of RON versus C₅.sup.⊕ liquid yield (wt%) for the conventional and PtIr₂ cluster catalysts listed in Table XI. At a given RON value the conventional catalyst demonstrated about a 3% higher C₅.sup.⊕ liquid yield than the PtIr₂ cluster catalyst. The liquid yield advantage exhibited by the conventional catalyst is significant. The higher naphtha cracking activity displayed by the PtIr₂ cluster catalyst is in good agreement with the higher C₄.sup.⊕ selectivity demonstrated by this catalyst for n-heptane dehydrocyclization. Since the conventional and PtIr₂ cluster catalysts contain the same metal and chloride concentration, the higher activity and enhanced cracking activity shown by the PtIr₂ cluster catalyst must be related to differences in catalyst microstructure.

Catalytic Reforming Over 1:6 Pt-Ir Catalysts

Naphtha reforming patterns over 0.4% Ir/0.067%-Pt/0.7% Cl/Al₂ O₃ catalysts are summarized in Tables VIII and IX. The activity patterns were found to be quite reproducible from preparation to preparation. The PtIr₆ cluster catalyst data indicate that no measurable catalytic effect is produced upon changing from 10/20 mesh to 1/16 inch extrudate catalyst particles. A lined out PtIr₆ cluster catalyst was found to be at least 2.4 times more active for naphtha reforming than a conventional 1:6 Pt-Ir catalyst (see FIG. 11). The composite RCA versus time curve exhibited by the PtIr₆ cluster catalyst clearly shows an increase in activity between 60 and 120 hours on feed. This behavior has been ascribed to the gradual loss of sulfur from the catalyst. After about 140 hours on feed the conventional and PtIr₆ cluster catalysts maintain fairly flat activity curves. These activity plateaus suggest that the catalysts have reached a steady state sulfur concentration. The shape of the PtIr₆ cluster catalyst curve further suggests that this catalyst may be able to tolerate feedstocks containing higher sulfur contents. Since the spent catalysts were found to contain nearly equivalent chloride contents, the large activity advantage exhibited by the PtIr₆ cluster catalyst cannot be correlated with a catalyst acidity difference. The activity advantage must reflect differences in catalyst microstructure. The bimetallic clusters present in the PtIr₆ cluster catalyst appear to be better suited to aromatize a naphtha feedstock. The suggested enhanced aromatization activity implies that the PtIr₆ cluster catalyst is intrinsically more efficient in carrying out demanding dehydrocyclization transformations. The higher toluene selectivity demonstrated by the PtIr₆ cluster catalyst (compared to a conventional 1:6 Pt-Ir catalyst) for n-heptane dehydrocyclization lends credence to this argument. The 30% lower coking rate displayed by the PtIr₆ cluster catalyst suggests that its useful lifetime will be considerably longer than the conventional catalyst.

As the activity of reforming catalysts increases, the yield of C₅.sup.⊕ reformate generally decreases. The decreased C₅.sup.⊕ liquid yield is the direct result of a higher cracking activity. During the catalytic reforming runs summarized in Tables VIII and IX and C₅.sup.⊕ liquid yield was monitored. FIG. 12 compares curves of RON versus C₅.sup.⊕ liquid yield (wt%) for PtIr₆ cluster and conventional 1:6 Pt-Ir catalysts. At a given RON value the conventional and PtIr₆ cluster catalysts appear to produce equivalent C₅.sup.⊕ liquid yields. The curve clearly shows that C₅.sup.⊕ liquid yields are highly dependent upon RON values. Since the individual runs were carried out under nearly identical conditions, RON values are a direct measure of catalyst activity.

Catalytic Reforming Over 1:2 Pt-Ir Catalysts

The results of naphtha reforming over 0.32% Ir/0.16% Pt/0.7% Cl/Al₂ O₃ catalysts are presented in Table XI. To minimize any diffusional effects both catalysts were in the form of 10/20 mesh particles. The conventional and PtIr₂ cluster catalysts were found to line out after about 200 hours on feed (see FIG. 13). The PtIr₂ cluster catalyst displayed at least a 1.6 fold activity advantage over the conventional 1:2 Pt-Ir catalyst. The increase in activity between 100 and 200 hours on feed exhibited by the PtIr₂ cluster catalyst may be associated with a gradual loss of sulfur from the catalyst. At a given RON value the conventional catalyst was found to produce about a 3% higher C₅.sup.⊕ liquid yield than the PtIr₂ cluster catalyst (see FIG. 14). The magnitude of the liquid yield advantage shown by the conventional catalyst is significant. The higher naphtha cracking activity demonstrated by the PtIr₂ cluster catalyst is in good agreement with the higher C₄.sup.⊕ selectivity exhibited by this catalyst in n-heptane conversion experiments. Since the conventional and PtIr₂ cluster catalysts contain similar metal and chloride loadings the enhanced cracking activity shown by the PtIr₂ cluster catalyst must somehow reflect different metal catalyzed hydrocracking activities. The relatively poor liquid yields obtained with a PtIr₂ cluster catalyst may be improved by using a higher sulfur content naphtha. Since the PtIr₂ cluster catalyst displays a 1.6 fold activity advantage over a conventional 1:2 Pt-Ir catalyst, it is possible to use a higher sulfur level without seriously decreasing the activity edge.

Summary of Naphtha Reforming Patterns

PtIr₂ and PtIr₆ cluster catalysts have been found to be at least 1.6 and 2.4 times more active for naphtha reforming than conventional 1:2 and 1:6 Pt-Ir catalysts, respectively. A PtIr₆ cluster catalyst was found to be 1.8 times more active than a PtIr₂ cluster catalyst. The activity advantage shown by the PtIr₆ cluster cannot be accounted for by the difference in Pt content. Liquid yields are comparable over PtIr₆ cluster and conventional 1:6 Pt-Ir catalysts. A PtIr₂ cluster catalyst was, however, found to produce a significantly lower (˜3%) C₅.sup.⊕ yield than a conventional 1:2 Pt-Ir catalyst. The higher naphtha reforming activities and liquid yield differences have been ascribed to the presence of unique heterometallic sites on cluster catalysts. These sites appear to be more efficient in carrying out aromatization reactions than those present on conventional Pt-Ir catalysts. The generation of such heteronuclear sites may only be possible by the use of preformed, heteronuclear noble metal cluster complexes as supported catalyst precursors. The significantly different catalytic reforming activities displayed by PtIr₂ and PtIr₆ cluster catalysts further suggests that catalysts can be radically altered by changing the stoichiometry and molecular structure of the precursor complexes.

                                      TABLE I                                      __________________________________________________________________________      INFRARED BANDS (cm.sup.-1) IN THE CARBONYL STRETCHING REGION                  FOR Pt/Ir, Pt/Ru AND Rh/Ir CLUSTER COMPLEXES                                   Compound            Nujol Mull.sup.(a)                                                                     THF Solution.sup.(b)                               __________________________________________________________________________     1. (pyridine).sub.2 Pt[Ir.sub.6 (CO).sub.15 ]                                                      2024 vs 2044 m                                                                 1976 vs 2022 sh                                                                1949 sh 2011 vs                                                                1730 s, br                                                                             1987 sh                                                                        1975 vs                                                                        1785 m                                                                         1733 m                                              Na.sub.2 [Ir.sub.6 (CO).sub.15 ]                                                                          2041 sh                                                                        2014 sh                                                                        1965 vs                                                                        1930 sh                                                                        1783 m                                                                         1725 m                                              [(C.sub.2 H.sub.5).sub.4 N].sub.2 [Ir.sub.6 (CO).sub.15 ]                                                 2020 sh                                                                        1970 s                                                                         1910 sh                                                                        1770 s                                                                         1745 s                                             2. (pyridine).sub.2 Pt[Ir.sub.2 (CO).sub.7 ]                                                       2043 sh 2035 sh                                                                2011 sh 2009 sh                                                                1979 vs, br                                                                            1983 vs                                                                1934 sh 1938 sh                                                                1793 sh 1785 sh                                                                1729 s  1732 m-s                                            Na[Ir(CO).sub.4 ]          1896 vs                                                                        1863 sh                                            3. [(C.sub.6 H.sub.5 ).sub.3 P].sub.2 Pt[Ir(CO).sub.3 P(C.sub.6 H.sub.5).s     ub.3 ].sub.2        2014 sh 2014 sh                                                                1996 sh 1995 sh                                                                1964 vs 1982 sh                                                                1944 sh 1962 vs                                                                1902 m-s                                                                               1934 sh                                                                1796 w  1906 m-s                                                               1761 vw 1800 w                                                                         1764 sh                                             Na[Ir(CO).sub.3 P(C.sub.6 H.sub.5).sub.3 ]                                                                1867 vs                                                                        1820 s                                             4. (pyridine).sub.3 Pt[Ru.sub.3 (CO).sub.12 ]                                                              2050 sh                                                                        1999 vs                                                                        1977 sh                                                                        1962 sh                                                                        1945 m                                                                         1804 vw, br                                                                    1763 w                                              Na.sub.2 [Ru.sub.3 (CO).sub.12 ]                                                                          2033 w                                                                         1960 sh                                                                        1946 vs                                                                        1918 sh                                                                        1891 sh                                                                        1717 vw                                                                        1649 w                                             5. ((C.sub.6 H.sub.5).sub.3 P).sub.2 Rh(CO)[Ir(CO).sub.4 ]                                                 2042 w                                                                         1995 sh                                                                        1987 vs                                                                        1962 sh                                                                        1932 m                                             __________________________________________________________________________      .sup.(a) KBr salt plates.                                                      .sup.(b) Matched 0.10 mm KBr solution cells.                             

                  TABLE II                                                         ______________________________________                                         ANALYSES OF SELECTED                                                           Al.sub.2 O.sub.3 -SUPPORTED CATALYSTS.sup.(a)                                              Nominal Loading                                                                (%)        Found (%)                                                           Pt   Ir     Ru     Pt   Ir    Ru                                   ______________________________________                                         Standard Catalysts                                                             --            0.40   --     --   0.38 --                                       II            0.067  0.40   --   0.089                                                                               0.33  --                                 III           0.067  0.40   --   0.081                                                                               0.36  --                                 IV            0.16   0.32   --   0.17 0.31  --                                 V             0.19   --     0.30 0.18 --    0.30                               Heteronuclear Noble                                                            Metal Cluster                                                                  Catalysts                                                                      A.sup.(b)     --     0.40   --   --   0.37  --                                 B.sup.(c)     0.067  0.40   --   0.054                                                                               0.35  --                                 C.sup.(c)     0.067  0.40   --   0.034                                                                               0.38  --                                 D.sup.(c)     0.067  0.40   --   0.036                                                                               0.40  --                                 E.sup.(d)     0.16   0.32   --   0.20 0.32  --                                 F.sup.(e)     0.19   --     0.30 0.19 --    0.40                               ______________________________________                                          .sup.(a) Analyses performed by Analytical Information Division, Exxon          Research and Engineering Company.                                              .sup.(b) [(C.sub.2 H.sub.5).sub.4 N].sub.2 [ Ir.sub.6 (CO).sub.15 ]            precursor                                                                      .sup.(c) (C.sub.5 H.sub.5 N).sub.2 Pt[Ir.sub.6 (CO).sub.15 ] precursor?        .sup.(d) (C.sub.5 H.sub.5 N).sub.2 Pt[Ir.sub.2 (CO).sub.7 ] precursor?         .sup.(e) (C.sub.5 H.sub.5 N).sub.3 Pt[Ru.sub.3 (CO).sub.12 ] precursor   

                                      TABLE III                                    __________________________________________________________________________     INFRARED BANDS (cm.sup.-1) IN THE CARBONYL STRETCHING                          REGION FOR Pt/Ir AND Pt/Ru CLUSTER COMPLEXES                                   SUPPORTED ON ALUMINA                                                           Catalyst                                                                            Metals (Wt %).sup.(a)                                                                     Precursors.sup.(b)                                                                           Carbonyl Bands.sup.(c)                           __________________________________________________________________________     1. D Ir (0.4)   Py.sub.2 Pt[Ir.sub.6 (CO).sub.15 ]                                                           2064 sh                                               Pt (0.067)               2046 sh                                                                        2024 vs                                                                        2009 sh                                                                        1968 sh                                                                        1804 w-m                                         2. A Ir (0.4)   [(C.sub.2 H.sub.5).sub.4 N].sub.2 [Ir.sub.6 (CO).sub.15                                      2014 s                                                                         1987 vs                                                                        1956 sh                                                                        1926 sh                                                                        1800 w                                                                         1770 w, br                                       3. E Ir (0.32)  Py.sub.2 Pt[Ir.sub.2 (CO).sub.7 ]                                                            2065 w                                                Pt (0.16)                2018 sh                                                                        2007 vs                                                                        1979 sh                                                                        1966 sh                                                                        1798 w-m                                                                       1755 sh                                          4. G Ir (2.0)   K[Ir(CO).sub.4 ]                                                                             2073 sh                                                                        2035 sh                                                                        1996 vs                                                                        1926 vs                                                                        1892 vs                                                                        1868 sh                                          5. H Ru (0.33)  Py.sub.3 Pt[Ru.sub.3 (CO).sub.12 ]                                                           2057 w                                                Pt (0.22)                2001 vs                                                                        1979 sh                                                                        1954 sh                                                                        1800 w, br                                       6. J Ru (2.0)   K.sub.2 [Ru.sub.3 (CO).sub.12 ]                                                              2086 w                                                                         2029 sh                                                                        1997 vs, br                                                                    1954 sh                                                                        1860 w                                                                         1804 w                                           __________________________________________________________________________      .sup.(a) Nominal metal loadings                                                .sup.(b) (Py) = pyridine                                                       .sup.(c) Spectra recorded as nujol mulls between KBr salt plates.        

                                      TABLE IV                                     __________________________________________________________________________     CHARACTERIZATION OF Pt/Ir/Al.sub.2 O.sub.3 CATALYSTS:                          APPARENT ACTIVE METAL SURFACE AREAS                                                          Chlorine         Impregnation.sup.(b)                            Catalyst                                                                            Metals (Wt %).sup.(a)                                                                   (Wt %)                                                                              Precursors  Method  H/M.sup.(c)                             __________________________________________________________________________      1. VI                                                                              Ir (1.2) 1.4  H.sub.2 IrCl.sub.6                                                                         I.W. (H.sub.2 O)                                                                       1.1                                          Pt (0.2)      H.sub.2 PtCl.sub.6                                           2. VII                                                                             Ir (0.4) 0.47 H.sub.2 IrCl.sub.6                                                                         I.W. (H.sub.2 O)                                                                       1.6                                          Pt (0.067)    H.sub.2 PtCl.sub.6                                           3. III                                                                             Ir (0.4) 0.70 H.sub.2 IrCl.sub.6                                                                         ADS (H.sub.2 O)                                                                        1.7                                          Pt (0.067)    H.sub.2 PtCl.sub.6                                           4. II                                                                              Ir (0.4) 0.70 H.sub.2 IrCl.sub.6                                                                         ADS (H.sub.2 O)                                                                        1.8                                          Pt (0.067)    H.sub.2 PtCl.sub.6                                           5. I                                                                               Ir (0.4) 0.70 H.sub.2 IrCl.sub.6                                                                         ADS (H.sub.2 O)                                                                        1.8                                      6. VIII                                                                            Ir (0.32)                                                                               0.48 H.sub.2 IrCl.sub.6                                                                         I.W. (H.sub.2 O)                                                                       1.5                                          Pt (0.16)     H.sub.2 PtCl.sub.6                                           7. IV                                                                              Ir (0.32)                                                                               0.70 H.sub.2 IrCl.sub.6                                                                         I.W. (H.sub.2 O)                                                                       1.7                                          Pt (0.16)     H.sub.2 PtCl.sub.6                                           8. K                                                                               Ir (1.2) 0.0  Py.sub.2 Pt[Ir.sub.6 (CO).sub.15 ]                                                         I.W. (THF)                                                                             0.80                                         Pt (0.2)                                                                   9. L                                                                               Ir (0.4) 0.0  Py.sub.2 Pt[Ir.sub.6 (CO).sub.15 ]                                                         I.W. (THF)                                                                             0.90                                         Pt (0.067)                                                                10. D                                                                               Ir (0.4) 0.70 Py.sub.2 Pt[Ir.sub.6 (CO).sub.15 ]                                                         I.W. (THF)                                                                             1.0                                          Pt (0.067)                                                                11. C                                                                               Ir (0.4) 0.70 Py.sub.2 Pt[Ir.sub.6 (CO).sub.15 ]                                                         ADS (THF)                                                                              1.1                                          Pt (0.067)                                                                12. B                                                                               Ir (0.4) 0.70 Py.sub.2 Pt[Ir.sub.6 (CO).sub.15 ]                                                         ADS (THF)                                                                              1.0                                          Pt (0.067)                                                                13. A                                                                               Ir (0.4) 0.70 [(C.sub.2 H.sub.5).sub.4 N].sub.2 [Ir.sub.6 (CO).sub.15                         ]          I.W. (CH.sub.3 OH)                                                                     0.85                                    14. M                                                                               Ir (0.32)                                                                               0.0  Py.sub.2 Pt[Ir.sub.2 (CO).sub.7 ]                                                          I.W. (THF)                                                                             0.95                                         Pt (0.16)                                                                 15. E                                                                               Ir (0.32)                                                                               0.70 Py.sub.2 Pt[Ir.sub.2 (CO).sub.7 ]                                                          I.W. (THF)                                                                             0.95                                         Pt (0.16)                                                                 __________________________________________________________________________      .sup.(a) In all cases analyzed, the actual metal loadings were in good         agreement with the nominal values.                                             .sup.(b) I.W. = incipient wetness, ADS = adsorption from an overstanding       solution.                                                                      .sup.(c) H/M = the ratio of hydrogen atoms absorbed to total number of         metal atoms present on catalyst.                                         

                                      TABLE V                                      __________________________________________________________________________     COMPARISON OF 0.4% Ir/0.067% Pt/0.47% Cl/Al.sub.2 O.sub.3 CATALYSTS:           n-HEPTANE DEHYDROCYCLIZATION                                                   Conditions: 900° F., 200 psig, W/hr/W = 20.0                            H.sub.2 /C.sub.7 = 5.6, 2.74 gm Catalyst                                               hr on                                                                               Conversion                                                                            Selectivities (%).sup.(c)                                                                    Coke.sup.(d)                                 Catalyst                                                                               feed (%)    Toluene                                                                              C.sub.4 --                                                                         i-C.sub.7                                                                          (%)                                          __________________________________________________________________________     VII.sup.(a)                                                                            22   92.7   14.2  60.9                                                                               4.01                                                     39   86.8   14.2  51.3                                                                               7.96                                                     47   88.3   21.3  46.6                                                                               8.35                                                     48   88.8   22.8  43.5                                                                               8.42                                                     63   75.0   25.4  24.9                                                                               19.1                                                     64   77.2   25.1  25.6                                                                               18.9                                                     69   76.8   24.2  24.6                                                                               19.4                                                     70   76.4   24.7  24.8                                                                               19.8                                                     87   73.8   25.7  23.0                                                                               20.6                                                     88   73.6   24.9  22.9                                                                               20.9                                                     92   73.0   24.8  22.1                                                                               21.0                                                     94   73.2   24.1  23.1                                                                               22.0                                                     112  72.1   22.5  23.0                                                                               23.7                                                     113  72.2   24.6  21.9                                                                               22.7                                                     114  72.7   24.3  21.5                                                                               22.6                                                     137  71.6   23.8  22.1                                                                               23.4                                                     144  68.1   24.4  20.7                                                                               24.9                                                     159  67.2   22.3  21.6                                                                               26.6                                                     165  65.0   23.1  19.9                                                                               27.3                                                     167  65.6   22.2  20.7                                                                               27.4                                                     183  66.2   20.4  22.1                                                                               27.7                                                     189  65.8   21.4  21.0                                                                               27.3                                                     207  67.1   21.8  21.3                                                                               26.8                                                     209  67.4   21.7  20.6                                                                               26.2                                                                               1.20                                         L.sup.(b)                                                                              37   81.2   22.7  36.0                                                                               13.3                                             plus 0.47% Cl                                                                          49   72.9   24.4  24.6                                                                               19.6                                                     61   73.8   24.3  24.6                                                                               19.7                                                     63   71.5   24.0  23.9                                                                               21.1                                                     89   72.5   24.9  23.9                                                                               20.2                                                     91   73.3   24.0  23.4                                                                               20.3                                                     110  77.4   24.6  26.6                                                                               17.1                                                     112  75.7   26.2  27.6                                                                               18.7                                                     114  78.0   24.5  27.0                                                                               16.6                                                     115  78.1   24.4  27.0                                                                               16.5                                                     116  77.9   24.3  27.0                                                                               16.6                                                     132  78.8   25.0  27.7                                                                               15.6                                                     135  77.6   25.3  27.1                                                                               16.3                                                     157  74.1   25.3  24.7                                                                               19.2                                                     158  74.7   24.0  25.3                                                                               19.2                                                     163  74.8   26.1  24.7                                                                               19.3                                                     180  72.7   25.7  24.3                                                                               19.8                                                     185  72.1   25.5  24.0                                                                               20.2                                                     186  72.7   25.7  24.3                                                                               19.8                                                     187  73.3   25.0  24.0                                                                               19.6                                                     205  73.4   24.7  25.1                                                                               20.1                                                     206  73.2   25.1  24.6                                                                               20.1                                                                               0.71                                          .sup.(a) Standard catalyst: Prepared by aqueous impregnation of Al.sub.2       O.sub.3 with chloroiridic and chloroplatinic acids.                            .sup.(b) Cluster catalyst: Prepared by impregnation of Al.sub.2 O.sub.3        with a tetrahydrofuran solution of (pyridine).sub.2 Pt[ Ir.sub.6               (CO).sub.15 ].                                                                 .sup.(c) Selectivities are based upon the fraction of nheptane converted.      .sup.(d) Spent catalysts were analyzed for coke after 210 hours on feed. 

                                      TABLE VI                                     __________________________________________________________________________     COMPARISON OF 0.32% Ir/0.16% Pt/η-Al.sub.2 O.sub.3 CATALYSTS:              n-HEPTANE DEHYDROCYCLIZATION                                                   Conditions: 900° F., 200 psig, W/hr/W = 20.0                            H.sub.2 /C.sub.7 = 5.0, 1.0 gm Catalyst                                               hr on                                                                              Conversion                                                                            Selectivities (%).sup.(d)                                                                     Coke.sup.(e)                                  Catalyst                                                                              feed                                                                               (%)    Toluene                                                                              C.sub.4.sup.-                                                                      i-C.sub.7                                                                           (%)                                           __________________________________________________________________________     VIII.sup.(a)                                                                          22.75                                                                              71.8   19.8  30.6                                                                               31.2                                                      24.25                                                                              70.1   19.2  28.4                                                                               33.4                                                      44.00                                                                              65.6   17.9  28.8                                                                               36.9                                                      49.75                                                                              63.9   18.1  27.7                                                                               36.8                                                      65.25                                                                              63.9   17.1  25.2                                                                               41.7                                                      69.50                                                                              63.1   16.8  24.9                                                                               41.7                                                      93.75                                                                              60.3   16.6  24.8                                                                               42.1 1.2                                           M.sup.(b)                                                                             20.75                                                                              67.8   19.0  32.7                                                                               26.7                                                      22.75                                                                              67.6   17.7  35.9                                                                               26.5                                                      44.25                                                                              61.3   18.5  31.0                                                                               31.5                                                      49.00                                                                              60.5   18.0  30.8                                                                               32.3                                                      68.00                                                                              58.6   17.3  31.1                                                                               32.9                                                      75.00                                                                              57.8   17.9  30.1                                                                               33.5                                                      93.00                                                                              56.1   17.2  30.2                                                                               34.3 1.5                                           M.sup.(c)                                                                             18.00                                                                              73.3   19.8  31.4                                                                               28.0                                               plus 0.5% Cl                                                                          20.75                                                                              70.5   19.5  30.4                                                                               29.6                                                      40.00                                                                              66.1   17.5  31.5                                                                               34.1                                                      49.75                                                                              65.9   17.4  31.4                                                                               34.1                                                      64.75                                                                              61.0   17.1  31.5                                                                               38.5                                                      69.25                                                                              62.8   17.6  29.0                                                                               35.5                                                      97.50                                                                              61.4   17.6  31.9                                                                               35.2 2.1                                           __________________________________________________________________________      .sup.(a) Standard catalyst: Prepared by aqueous impregnation of Al.sub.2       O.sub.3 with chloroiridic and chloroplatinic acids.                            .sup.(b) Cluster catalyst: Prepared by impregnation of Al.sub.2 O.sub.3        with a tetrahydrofuran solution of (pyridine).sub.2 Pt[Ir.sub.2 (CO).sub.      ].                                                                             .sup.(c) Cluster catalyst + 0.5% chloride: Prepared by treating M with         aqueous HCl.                                                                   .sup.(d) Selectivities based upon fraction of nheptane converted.              .sup.(e) The spent catalysts were analyzed for coke after 100 hours on         feed.                                                                    

                  TABLE VII                                                        ______________________________________                                         n-HEPTANE DEHYDROCYCLIZATION EMPLOYING A                                       0.33% Ru/0.22% Pt/Al.sub.2 O.sub.3 CATALYST                                    Conditions: 900° F., 200 psig, W/hr/W = 20                              H.sub.2 /C.sub.7 = 5.0, 1.0 gm Catalyst                                        hr on      Conversion                                                                               Selectivities (%).sup.(b)                                 Catalyst                                                                              feed    (%)       Toluene  C.sub.4 --                                                                            i-C.sub.7                             ______________________________________                                         H.sup.(a)                                                                             22.5    55.9      2.83     50.5   38.1                                         42.5    41.28     2.76     29.1   59.9                                         46.5.sup.(c)                                                                           58.7      5.66     29.1   52.1                                  ______________________________________                                          .sup.(a) Cluster catalyst: Prepared by impregnating Al.sub.2 O.sub.3 with      a tetrahydrofuran solution of (pyridine).sub.3 Pt[Ru.sub.3 (CO).sub.12 ].      .sup.(b) Selectivities are based upon the fraction of nheptane converted.      .sup.(c) Reaction temperature increased to 950° F.                

                  TABLE VIII                                                       ______________________________________                                         COMPARISON OF 0.4% Ir/0.067% Pt/0.7 Cl/Al.sub.2 O.sub.3                        CATALYSTS: BAYWAY NAPHTHA REFORMING.sup.(a)                                    Conditions: 200 psig, w/hr/w = 2.26                                            6000 SCF H.sub.2 /BBL, 5.0 gm. catalyst                                               Time                         (%)     (%)                                Catalyst                                                                              (hr)     T°F.                                                                           RON   RCA    Coke,   Cl.sup.(d)                         ______________________________________                                         III.sup.(b)                                                                            0-23    907    106.0 380                                                      23-47    910    104.7 259                                                      47-71    912    103.0 177                                                      71-92    909    102.5 167                                                       92-117  --     --    --                                                       117-143  909    104.9 281                                                      143-167  910    104.3 238                                                      167-191  910    103.2 186                                                      191-215  910    103.0 180                                                      215-239  910    103.2 186                                                      239-264  912    103.5 188                                                      264-288  914    104.0 192    1.34    0.59                               D.sup.(c)                                                                              0-18    905    104.7 298                                                      18-50    910    104.8 264                                                      50-70    905    104.2 269                                                      70-94    907    104.4 269                                                       94-118  907    105.0 307                                                      118-142  905    105.3 342                                                      142-166  905    105.3 342                                                      166-192  910    106.7 410                                                      192- 216 910    106.7 410                                                      216-238  907    106.3 404                                                      238-262  909    106.7 427                                                      262-286  889    103.9 411    0.94    0.52                               ______________________________________                                          .sup.(a) The Bayway naphtha is characterized as follows: 47.2% paraffins,      42.2% naphthenes and 10.5% aromatics, .sup.d 75° F. = 0.7539, RON       54, 0.5 ppm sulfur (by thiophene addition).                                    .sup.(b) Standard catalyst was prepared by the impregnation of 1/16 inch       Al.sub.2 O.sub.3 extrudate (179 m.sup.2 /gm) with chloroiridic and             chloroplatinic acid solutions. The chlorine content was adjusted to 0.7        wt. % by the addition of aqueous HCl.                                          .sup.(c) Cluster catalyst was prepared by the impregnation of a dry 0.7        wt. % Cl/Al.sub.2 O.sub.3 support (178 m.sup.2 /gm) with a tetrahydrofura      solution of (pyridine).sub.2 Pt[Ir.sub.6 (CO).sub.15 ]. The final catalys      was in the form of 10/20 mesh particles.                                       .sup.(d) The spent catalysts were analyzed for coke and chlorine content.

                  TABLE IX                                                         ______________________________________                                         COMPARISON OF 0.4% Ir/0.067% Pt/0.7% Cl/Al.sub.2 O.sub.3                       CATALYSTS: BAYWAY NAPHTHA REFORMING.sup.(a)                                    Conditions: 200 psig, w/hr/w = 2.26                                            6000 SCF H.sub.2 /BBL, 5.0 gm catalyst                                                   Time                                                                 Catalyst  (hr)       T° F.                                                                             RON     RCA                                     ______________________________________                                         II.sup.(b)                                                                                0-20      909       104.0   229                                               20-44      --        --      --                                                44-67      907       102.6   177                                               67-90      909       102.5   165                                                90-117    907       102.4   170                                               117-139    910       102.0   143                                               139-165    910       102.3   152                                               165-189    912       103.3   179                                     B.sup.(c)  0-19      905       106.3   428                                               19-39      907       104.9   293                                               39-71      907       103.9   255                                               71-91      905       104.2   269                                                91-115    909       106.0   354                                               115-140    918       107.4   411                                               140-164    909       106.7   427                                               164-188    901       105.7   421                                     ______________________________________                                          .sup.(a) Bayway feed: 47.2% paraffins, 42.2% naphthenes, 10.5% aromatics,      d.sub. 75° F. = 0.7539, RON = 54, 0.5 ppm total sulfur                  .sup.(b) Standard catalyst was prepared by impregnation of 1/16 inch           Al.sub.2 O.sub.3 extrudate (179 m.sup.2 /gm) with chloroiridic and             chloroplatinic acid solutions. The chlorine content was adjusted to 0.7        wt. % by the addition of aqueous HCl.                                          .sup.(c) Cluster catalyst was prepared by the impregnation of a dry 0.7        wt. % Cl/Al.sub.2 O.sub.3 extrudate (179 m.sup.2 /gm) with a                   tetrahydrofuran solution of (pyridine).sub.2 Pt[Ir.sub.6 (CO).sub.15 ].  

                  TABLE X                                                          ______________________________________                                         COMPARISON OF 0.4% Ir/0.7% Cl/Al.sub.2 O.sub.3 CATALYSTS:                      BAYWAY NAPHTHA REFORMING.sup.(a)                                               Conditions: 200 psig, W/hr/W = 2.26                                            6000 SCF H.sub.2 /BBL, 5.0 gm Catalyst                                                Time                            (%)  (%)                                Catalyst                                                                              (hr)       T°F.                                                                            RON     RCA  Coke Cl.sup.(d)                         ______________________________________                                         I.sup.(b)                                                                              0-23      910     106.sup.+                                                                              --                                                  17-46      910     106.3   360                                                 46-70      910     103.5   198                                                 70-93      910     103.1   182                                                  93-116    909     103.4   202                                                 116-140    910     103.7   205                                                 140-166    909     104.2   224                                                 166-190    909     104.5   258                                                 190-214    912     105.1   263  1.28 0.66                               A.sup.(c)                                                                              0-23      905     106.7   370                                                 23-43      905     104.3   215                                                 43-68      905     104.1   202                                                 68-92      912     104.3   177                                                  92-116    912     105.2   216                                                 116-140    910     103.8   166                                                 140-164    912     103.0   132                                                 164-188    910     103.0   139                                                 188-213    914      99.7    62                                                 213-236    909      98.4    58                                                 236- 259   910      99.3    64  1.54 0.65                               ______________________________________                                          .sup.(a) Bayway feed: 47.2% paraffins, 42.2% naphthenes, 10.5% aromatics,      .sup.d 75° F. = 0.7539, RON = 54, 0.5 ppm total sulfur.                 .sup.(b) Standard catalyst was prepared by impregnating 1/16 inch Al.sub.      O.sub.3 extrudate (178 m.sup.2 /gm) with chloroiridic acid solution. The       chlorine content was adjusted to 0.7 wt. % by the addition of aqueous HCl      .sup.(c) Cluster catalyst was prepared by impregnating a dry 0.7 wt. %         Cl/Al.sub.2 O.sub.3 support (167 m.sup.2 /gm) with a methanol solution of      [(C.sub.2 H.sub.5).sub.4 N].sub.2 [Ir.sub.6 (CO).sub.15 ].                     .sup.(d) Spent catalysts were analyzed for wt. % carbon and chlorine.    

                  TABLE XI                                                         ______________________________________                                         COMPARISON OF                                                                  0.32 Ir/0.16% Pt/0.7% Cl/Al.sub.2 O.sub.3 CATALYSTS:                           BAYWAY NAPHTHA REFORMING.sup.(a)                                               Conditions: 200 psig, W/hr/W = 2.26                                            6000 SCF H.sub.2 /BBL, 5.0 gm Catalyst                                                Time                            %.sup.(e)                                                                           %.sup.(d)                          Catalyst                                                                              (hr)      T°F.                                                                             RON    RCA   Coke Cl                                 ______________________________________                                         IV.sup.(b)                                                                             0-20     910      106.0  340                                                  20-45     909      104.9  281                                                  45-69     909      104.7  269                                                  69-89     910      103.8  212                                                   89-114   912      103.9  205                                                  114-141   910      103.5  198                                                  141-164   909      103.3  197                                                  164-185   907      102.2  161                                                  185-213   907      101.9  152                                                  213-237   907      101.6  148                                                  237-257   910      101.8  137                                                  257-282   909      101.7  139                                                  282-309   909      102.0  149                                                  309-332   910      102.1  147   1.50 0.42                               E.sup.(c)                                                                              0-21     909      104.8  269                                                  21-45     909      105.2  302                                                  45-69     907      103.5  215                                                  69-93     909      103.3  197                                                   93-113   910      103.9  216                                                  113-138   914      103.5  179                                                  138-166   910      103.9  216                                                  l66-189   909      103.9  225                                                  189-214   907      103.0  193                                                  214-237   909      104.0  229                                                  237-261   910      104.3  238                                                  261-285   910      104.3  238                                                  285-306   910      104.4  243                                                  306-333   910      104.4  243   1.94 0.48                               ______________________________________                                          .sup.(a) Bayway feed: 47.2% paraffins, 42.2% naphthenes, 10.5% aromatics,      .sup.d 75° F. = 0.7539, RON = 54, 0.5 ppm total sulfur.                 .sup.(b) Standard catalysts was prepared by impregnating Al.sub.2 O.sub.3      powder (154 m.sup.2 /gm) with chloroiridic and chloroplatinic acid             solutions. The chlorine content was adjusted to 0.7 wt. % by the addition      of aqueous HCl.                                                                .sup.(c) Cluster catalyst was prepared by impregnating a dry                   0.7%/Cl/Al.sub.2 O.sub.3 powder (167 m.sup.2 /gm) with a tetrahydrofuran       solution of (pyridine).sub.2 Pt[Ir.sub.2 (CO).sub.7                            .sup.(d) Spent catalysts were analyzed for wt. % carbon and chlorine.     

What is claimed is:
 1. In a hydrocarbon conversion process wherein a selected hydrocarbon feedstream is contacted with a mixed noble metal catalyst under suitable conditions thereby effecting the desired change in the hydrocarbon, the improvement comprising using as the heteronuclear noble metal catalyst a supported heteronuclear noble metal cluster catalyst prepared by the reduction of supported heteronuclear noble metal cluster complexes which heteronuclear noble metal cluster complexes are selected from the group consisting of:(pyridine)₂ Pt[Ir₆ (CO)₁₅ ], (pyridine)₂ Pt[Ir₂ (CO)₇ ], ((C₆ H₅)₃ P)₂ Pt[Ir(CO)₃ P(C₆ H₅)₃ ]₂, (pyridine)₃ Pt[Ru₃ (CO)₁₂ ], ((C₆ H₅)₃ P)₂ Rh(CO)[Ir(CO)₄ ], and (pyridine)₂ Pt[Rh(CO)₂ (P(C₆ H₅)₃)₂ ]₂ and wherein the supports are selected from the group consisting of anhydrous refractory inorganic metal oxides and carbon.
 2. The process of claim 1 wherein the catalyst further comprises from 0.001 to 3.0 wt. % chlorine and/or fluorine.
 3. The process of claim 1 wherein the support is alumina.
 4. The process of claim 2 wherein the support is alumina.
 5. The process of claim 1 wherein the total noble heterometal loading of the catalyst is within the range of 0.002 to 35 wt. % based on total catalyst weight.
 6. The process of claim 1 wherein the supported heteronuclear noble metal cluster catalyst is a supported PtIr₆ cluster catalyst prepared by the reduction of a supported (pyridine)₂ Pt[Ir₆ (CO)₁₅ ] complex.
 7. The process of claim 1 wherein the supported heteronuclear noble metal cluster catalyst is a supported PtIr₂ cluster catalyst prepared by the reduction of a supported (pyridine)₂ Pt[Ir₂ (CO)₇ ] complex.
 8. The process of claim 1 wherein the supported heteronuclear noble metal cluster catalyst is a supported PtIr₂ cluster catalyst prepared by the reduction of a supported ((C₆ H₅)₃ P)₂ Pt[Ir(CO)₃ P(C₆ H₅)]₂ complex.
 9. The process of claim 1 wherein the supported heteronuclear noble metal cluster catalyst is a supported PtRu₃ cluster catalyst prepared by the reduction of a supported (pyridine)₃ Pt[Ru₃ (CO)₁₂ ] complex.
 10. The process of claim 1 wherein the supported heteronuclear noble metal cluster catalyst is a supported RhIr cluster catalyst prepared by the reduction of a supported ((C₆ H₅)₃ P)₂ Rh(CO)[Ir(CO)₄ ] complex.
 11. The process of claim 1 wherein the supported heteronuclear noble metal cluster catalyst is a supported PtRh₂ cluster catalyst prepared by the reduction of a supported (pyridine)₂ Pt[Rh(CO)₂ (P(C₆ H₅)₃)₂ ]₂ complex.
 12. The process according to claims 6, 7, 8, 9, 10 or 11 wherein the catalyst further comprises from 0.001 to 3.0 wt.% of a halogen selected from the group consisting of chlorine, fluorine and mixtures thereof. 